Chemical Principles of Hydrogen Production by Electrolysis of Water

In industry, the following methods are usually used to produce hydrogen; one is to pass water vapor through hot coke (carbon reduction method) to obtain hydrogen with a purity of about 75%; the other is to pass water vapor through hot iron to obtain a purity of The third method is to extract hydrogen from water gas, and the purity of the hydrogen obtained is also low; the fourth method is the electrolysis of water method, and the purity of the hydrogen produced can be as high as 99% or more. This is a method for preparing hydrogen in industry. an important method. During the electrolysis of sodium (potassium) hydroxide solution, oxygen is released at the anode and hydrogen is released at the cathode. Hydrogen can also be obtained by electrolyzing sodium chloride aqueous solution to produce sodium hydroxide.

The so-called electrolysis is the process of decomposing electrolytes dissolved in water into new substances with the help of direct current. When direct current is passed into some electrolyte aqueous solutions, the decomposed substances have nothing to do with the original electrolyte. What is decomposed is water as the solvent, and the original electrolyte remains in the water. For example, sulfuric acid, sodium hydroxide, potassium hydroxide, etc. all belong to this type of electrolytes.

When electrolyzing water, since pure water has very little ionization and low conductivity, it is a typical weak electrolyte. Therefore, the aforementioned electrolytes need to be added to increase the conductivity of the solution so that water can be smoothly electrolyzed into hydrogen and oxygen. Electrolytes such as potassium hydroxide will not be electrolyzed. Let's take potassium hydroxide as an example:

(1) Potassium hydroxide is a strong electrolyte. When dissolved in water, the following ionization process occurs:

KOH - K+tenOH-

As a result, a large amount of K+ and OH- were produced in the aqueous solution.

(2) The reactivity of metal ions in aqueous solutions is different. They can be arranged in order of reactivity as follows:

K>Na>Mg>Al>Mn>Zn>Fe>Ni>Sn>Pb>H>Cu>Hg>Ag>Au

In the arrangement above, the metal in the front is more lively than the one in the back.

(3) In the order of metal activity, the more active the metal, the easier it is to lose electrons, otherwise the opposite is true. From an electrochemical theory point of view, metal ions that can easily obtain electrons have a high electrode potential, while metal ions ranked first in the order of activity are difficult to obtain electrons and become atoms due to their low electrode potential. The electrode potential of hydrogen ions is -1.71V, while the electrode potential of potassium ions is -2.66V. Therefore, when hydrogen ions and potassium ions exist in the aqueous solution at the same time, the hydrogen ions will first obtain electrons on the cathode and become hydrogen gas, and The potassium ions remain in solution.

(4) Water is a weak electrolyte and is difficult to ionize. When potassium hydroxide is dissolved in water, the ionized potassium ions are surrounded by polar water molecules and become hydrated potassium ions. The action of potassium ions makes the water molecules With polar direction. Under the action of direct current, potassium ions and hydrated molecules with polar directions move toward the cathode. At this time, hydrogen ions first gain electrons and become hydrogen gas. Therefore, in the electrolysis process using potassium hydroxide as the electrolyte, water is actually electrolyzed to produce hydrogen and oxygen, while potassium hydroxide only plays the role of carrying charges.

When electrolyzing water, the DC voltage applied to the electrolytic cell must be greater than the theoretical decomposition voltage of water in order to overcome various resistance voltage drops and electrode polarization electromotive force in the electrolytic cell. The electrode polarization electromotive force is the sum of the overpotential when hydrogen is precipitated at the cathode and the overpotential when oxygen is precipitated at the anode. Therefore, the water electrolysis voltage U can be expressed as: U=U0+IR+hydrogen overpotential+oxygen overpotential

In the formula, U0--Theoretical decomposition voltage of water, V;

I--Electrolysis current, A

R--Total resistance of electrolytic cell, Ω

From the perspective of energy consumption, the electrolysis voltage should be reduced as much as possible. The factors that affect electrolysis voltage mainly include the following three aspects:

(1) Theoretical decomposition voltage (approximately 1.23V at 0.1MPa and 25℃), which decreases with the increase of temperature and increases with the increase of pressure. For every 10 times increase in pressure, the voltage increases by approximately 43mV. 

(2) Hydrogen and oxygen overpotential. There are many factors that affect the overpotential of hydrogen and oxygen. First of all, the electrode material and the surface state of the electrode have a greater impact on it. For example, the hydrogen overpotential of iron and nickel is lower than that of lead, zinc, mercury, etc., and the oxygen overpotential of iron and nickel exceeds that of lead, zinc, and mercury. The potential is also lower than lead. The larger the contact area with the electrolyte or the rougher the electrode surface, the smaller the hydrogen and oxygen overpotential generated. Secondly, as the current density increases during electrolysis, the overpotential will increase accordingly, and the rise in temperature will also cause an increase in the overpotential. In addition, the overpotential is also related to factors such as the nature, concentration of the electrolyte, and impurities in the solution. For example, on a nickel electrode, the oxygen overpotential of a dilute solution is greater than that of a concentrated solution.

In order to reduce the hydrogen and oxygen overpotential, some methods can be adopted. Such as increasing the operating temperature and using appropriate electrode materials. In addition, appropriately increasing the actual surface area of ​​the electrode or roughening the electrode surface can reduce the electrode resistance and overpotential to varying degrees, thereby achieving the purpose of reducing the operating voltage.

(3) Resistor voltage drop. The total resistance in the electrolytic cell includes the resistance of the electrolyte, diaphragm resistance, electrode resistance and contact resistance, among which the first two are the main factors. Diaphragm resistance voltage drop depends on the thickness and properties of the material. Using a general asbestos diaphragm, when the current density is 2400A/m2, the voltage drop on the diaphragm resistor is about 0.25-0.30V. When the current density increases again, the voltage drop will increase to about 0.5V. The higher the conductivity of the electrolyte, the smaller the voltage drop in the electrolyte. For the electrolyte, in addition to having a small resistance value, it is also required that it does not decompose under the electrolysis voltage; it does not escape with hydrogen and oxygen due to volatilization; it is not corrosive to the electrolytic cell materials; when the pH value of the solution When changing, it should have certain buffering performance.

Most electrolytes are easy to decompose during electrolysis and should not be used when electrolyzing water. Sulfuric acid generates persulfuric acid and ozone at the anode, which is very corrosive and should not be used. Strong alkali can meet the above requirements, so KOH or NaOH aqueous solution is generally used as the electrolyte in industry. KOH has better electrical conductivity than NaOH, but it is more expensive. At higher temperatures, its corrosive effect on the electrolytic cell is stronger than that of NaOH. In the past, NaOH was often used as the electrolyte in our country. However, in view of the fact that the current electrolytic cell materials are already resistant to the corrosion of KOH, in order to save electric energy, there has been a general trend to use KOH solution as the electrolyte.In addition, during the process of electrolyzing water, the electrolyte will contain hydrogen and oxygen bubbles that are continuously precipitated, which increases the resistance of the electrolyte. The percentage of the volume of bubbles in the electrolyte to the volume of the electrolyte including bubbles is called the gas content of the electrolyte. The gas content is related to the current density during electrolysis, electrolyte viscosity, bubble size, working pressure and electrolytic cell structure and other factors. Increasing the circulation speed and working pressure of the electrolyte will reduce the gas content; increasing the current density or increasing the working temperature will increase the gas content. In actual situations, bubbles in the electrolyte are inevitable, so the resistance of the electrolyte will be much greater than without bubbles. When the gas content reaches 35%, the resistance of the electrolyte is twice that of when there are no bubbles. Reducing the operating voltage is conducive to reducing power consumption. For this reason, effective measures should be taken to reduce hydrogen and oxygen overpotentials and resistance voltage drops. In general, when the current is small, the former is the main factor; when the current is large, the latter will become the main factor.

When the electrolytic cell operates under high working pressure, the gas content of the electrolyte decreases, thereby reducing the resistance of the electrolyte. For this reason, an electrolytic cell that can operate under a pressure of 3MPa has been developed. However, the working pressure gauge should not be too high, otherwise it will increase the solubility of hydrogen and oxygen in the electrolyte, causing them to regenerate water through the diaphragm, thus reducing the current efficiency. Increasing the operating temperature can also reduce the resistance of the electrolyte, but the corrosion of the electrolyte by the electrolyte will also intensify. If the temperature is greater than 90°C, the electrolyte will cause serious damage to the asbestos diaphragm and form soluble silicates in the asbestos diaphragm. To this end, a variety of diaphragm materials for high-temperature corrosion have been developed, such as nickel powder metallurgy sheets and diaphragm materials bonded with potassium titanate fiber and polytetrafluoroethylene, which can be used in alkali solutions at 150°C. In order to reduce the resistance of the electrolyte, the current density of the electrolytic cell can also be adopted, the circulation speed of the electrolyte can be accelerated, and the distance between the electrodes can be appropriately reduced.

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